Frequency-controlled variable-speed drives

V ri ble Frecjuency Drives. An important energy by-product of soHd-state electronics is the relatively low cost variable speed drive. These electronic devices adjust the frequency of current to control motor speed such that a pump can be controlled direcdy to deUver the right flow without the need for a control valve and its inherent pressure drop. Eigure 11 shows that at rated load the variable speed drive uses only about 70% as much power as a standard throttle control valve system, and at half load, it uses only about 25% as much power. 

Feed tank and metering pump the flow rate through such a pump can be controlled by a stroke adjusting mechanism or a variable speed drive acting on the stoke frequency. Control can be achieved by a fixed adjustment or through a flow meter. 

When the drive (motor and control) performance requirements are minimal, a standard industrial AC induction motor can often be successfiiUy applied to adjustable-frequency power, variable-speed applications. Indeed, some applications can be converted from constant speed to variable speed while utilizing an existing induction motor. However, when the performance level required is more demanding, a definite-purpose motor design is appropriate. This is usually the case when maximum process productivity is the goal. 

Variable Air Flow Fans. Variable air flow fans are needed ia the process iadustry for steam or vapor condensing or other temperature critical duties. These also produce significant power saviags. Variable air flow is accompHshed by (/) variable speed motors (most commonly variable frequency drives (VFDs) (2) variable pitch fan hubs (J) two-speed motors (4) selectively turning off fans ia multiple fan iastaHations or (5) variable exit louvers or dampers. Of these methods, VFDs and variable pitch fans are the most efficient. Variable louvers, which throttle the airflow, are the least efficient. The various means of controlling air flow are summarized ia Table 3. 

Options for connecting the motor drive to the shaft depend on the shaft orientation. A vertical-shaft cantilever design would prefer a belt drive to reduce the cost of manufacture of the support structure and to facilitate maintenance. A horizontal shaft has the additional option of direct coupling. Variable speed can be accomplished through a gearbox or preferably through variable frequency control on the motor. In addition to the power requirements discussed previously, the startup power to overcome the torque of the rotor must be considered. This startup power is related to the time required to reach the desired rotor speed. 

DC motor drives have always offered high torque at all speeds and exact control of motion speed. AC induction motors have reliably converted electricity into rotary power for many years, and recently adjustable-frequency controls add variable-speed capability. While AC motors were originally relegated to relatively simple tasks, such as varying the flow rates of fans or pumps, advances in both motor and control technologies have allowed their use in higher performance operations. They are reliable sources of fixed-speed and variable-speed rotating power. Electric drives with appropriate closed-loop control operate only when required. However, to avoid unsuccessful apphca-tions, it is important to properly match the load, motor, and controller. 

During the 1980 s attention turned to AC machines technology with both synchronous and induction being successfully applied. Variable speed and torque control was made possible by variable rotor resistance in the form of Wound Rotor Induction Motors and then the Cyclo Convertor which applied thyristor technology to produce low frequency ac output suitable for the driving large AC machines. 

If the compressor is of the variable speed type then we can use this to control the flow, as shown in Figure 11.6. By changing the speed we move from one compressor curve to another. This is energy efficient and, because the surge point moves, can operate over a wide range. However variable frequency drives (VFD) for large electric motors are costly. Variable speed steam turbine drivers have a mixed reputation. Most success has been had with gas turbine drivers. 

One method of controlling the speed of ac motors is with a pulse-width-modulated (PWM) variable-frequency drive (VFD). In the simplest terms, the VFD converts ac line voltage into variable voltage and frequency, which is then used to control the speed of ac squirrel-cage motors. As a controller, start/stop, hand-off-auto, and various other control schemes may be applied. 

One industrial facility that has done exhaust system retrofit is the Eldec Corporation, an aerospace electronic manufacturer. With the help of the local utility, Eldec implemented a control project to reduce exhaust air by up to 30% for the first shift and 60% for the rest of the time and achieved great savings with one year simple payback. The project closed the exhaust inlets with dampers and controlled the exhaust fan speeds with variable frequency drives (VFD). The exhaust fans are now monitored and controlled by the building direct digital controls (DDC) system to ensure proper operation and save energy. 

Variable frequency drives (VFDs) are sometime used to adjust the operation of typical (US) standard of 480VAC, 3PH, 60hz operation of the motor. The functionality of a VFD is to convert frequency measured in Hertz (Hz) to motor speed. One Hz equals 1 cycle per second. When voltage is being received (input to the VFD), it is in the sinusoidal waveform. The sine wave is converted to a digital square wave that now controls the revolutions per minute (RPM) of the motor. 

Variable frequency speed control driving motor can be adopted for induced draft fan. 

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